Strontium ion (Sr²⁺) matches krypton’s noble gas configuration

Steps:

• Strontium (Sr) has atomic number 38; its neutral configuration is [Kr] 5s².
• The strontium ion is Sr²⁺, formed by losing two 5s electrons, resulting in [Kr] configuration.
• Krypton (Kr) has atomic number 36; its neutral atom has the stable [Ar] 3d¹⁰ 4s² 4p⁶ ([Kr]) configuration.
• Compare options: only krypton’s neutral atom matches Sr²⁺ exactly.

Why B is correct:

• Per noble gas rule, Sr²⁺ achieves the stable octet of krypton by removing valence electrons, identical to Kr’s ground-state configuration.

Why the others are wrong:

• A. Calcium (atomic number 20) neutral: [Ar] 4s², lacks Kr core.
• C. Rubidium (atomic number 37) neutral: [Kr] 5s¹, has extra 5s electron.
• D. Selenium (atomic number 34) neutral: [Ar] 3d¹⁰ 4s² 4p⁴, incomplete p subshell before Kr.

Final answer: B